This happens to me twice: I press the power button on my Mac Book Pro, I choose sleep, I close it, I unplug everything, I confirm that is off (by pressing my ear to it) and put it in my back. Some minutes latter it wakes up by itself. Both times I caught it in time, well, the second time it was so hot I couldn't touch some parts and it was refusing to actually wake up, the screen was blank. Restarting it worked though.

Any ideas what might be going on and/or how to prevent this?